This invention relates to materials of nonohmic resistors, namely, resistors having nonlinear voltage-current characteristics and, more particularly, to materials of nonohmic resistors for use in electric communication apparatus.
Nonohmic resistors (hereafter called varistors) have widely been used in surge-absorbing devices, voltage-stabilizing devices, lightning arrestors, and others. In general, a varistor is evaluated by a voltage nonlinear exponent .alpha. and a breakdown voltage V.sub.i used in an equation: EQU I/i = (V/V.sub.i).sup..alpha.,
where V.sub.i, named the breakdown voltage hereinabove, represents a voltage applied across the varistor to cause a predetermined current i to flow therethrough and I generally represents a current flowing through the varistor when a voltage V is applied thereacross.
Sophisticated varistors are silicon varistors, silicon-carbide varistors, selenium rectifiers, cuprous oxide rectifiers, and the like and have various defects such that they have small voltage nonlinear exponents .alpha., that their characteristics are hardly optionally adjustable or controllable, and/or that they are bulky. It has recently been proposed by Michio MATSUOKA in "Japanese Journal of Applied Physics," Vol. 10, No. 6 (June 1971), pages 736-746, to provide a varistor as a sintered body or mass of zinc oxide ceramics with five additives of bismuth(III) oxide, cobalt(II) oxide, manganese(II) oxide, chromium(III) oxide, and antimony(III) oxide and to thereby remove the above-mentioned defects and widen the field of application of the varistors. The proposed varistor, however, is still unsatisfactory for use in those circuits of recent highly developed electric communication apparatus in which a varistor is mostly used with a load imposed thereon as a surge-absorbing element for absorbing a large current pulse inevitably appearing in the circuit upon on-off of a switch for the circuit and under like circumstances. Furthermore, the proposed varistor is defective in that continued application thereto of a certain electric power results in a decrease in the breakdown voltage V.sub.i and a consequent increase in the leakage current. The proposed varistor likewise when supplied repeatedly with large current pulses does not tend to retain its excellent nonohmic characteristics.
Except for specific electric circuits, the voltage nonlinear exponent .alpha. of a varistor should be about 30 or more. The breakdown voltage V.sub.i should be adjustable to a nominal voltage preselected for the varistor in consideration of the usage thereof. Dielectric properties are also important when a varistor is used in communication apparatus. The varistor shows a small impedance for that frequency of a signal used in communication apparatus which is rendered more and more higher with the progress of communication techniques. It is therefore desirable that a varistor has a smaller capacitance or dielectric constant .epsilon.. It is also desired that a varistor has a smaller dielectric loss (tan.delta.). The conventional varistor comprising zinc oxide as its principal constituent has had as large as dielectric constant .epsilon. and a dielectric loss (tan.delta.) as about 1000 or more and 10% or more, respectively, and has been unsatisfactory for use in communication apparatus.
Proposals have also been made to render the varistors more resistive against the continued load and the large current pulses. One of the proposals is to make the varistor which consists mainly of zinc oxide to include various vitreous materials by one method or another. The improvements thereby achieved, however, have been either for the load or for the large current pulse. This proves that the mechanism of variation caused in the varistor characteristics by the load is different from the mechanism for the large current pulse and that the improvement has consequently been possible for only one of the load and the large current pulse. Furthermore, addition of the vitreous material has raised the breakdown voltage V.sub.i to about 500 volts or more. As a result, varistors for use in communication apparatus have to be rendered very thin in order to provide a compromise between the voltage of the signal used therein and the breakdown voltage so that the varistors do not further withstand a high surge voltage and that the varistors have only weak mechanical strength and unduly large capacitance.